Amplifier circuits have been used in various electronic devices such as audio devices. Amplifier circuits increase powers and/or amplitudes of signals. In many applications, power amplifier circuits are used at the output stage of a system to drive an external device. For example, an output power amplifier has been used to drive an external speaker. The output power amplifier may need to provide a gain from about −57 dB to about 6 dB for the audio system.
FIG. 1 is a conventional single-stage feedback amplifier. In FIG. 1, the single-stage feedback amplifier 100 includes an operational amplifier 110 and resistors having resistances r1 and r2. The single-stage feedback amplifier 100 provides a gain of about 20*log(r2/r1). In order to obtain a gain of about −57 dB, r2 (e.g., 140Ω) is much smaller than r1 (e.g., 100 kΩ). The low resistance of r2 results in a parasitic resistance at r2 due to layout mismatch and long metal routing and the operation of the single-stage feedback amplifier 100 suffers a gain offset. Additionally, a large current from the previous stage may be needed to drive the resistor having the low resistance r2.
In order to solve the issue described above, a conventional two-stage feedback amplifier is provided as shown in FIG. 2. In FIG. 2, the two-stage feedback amplifier 200 consists of two operational amplifiers 210 and 220 and resistors r1-r4. To cover the gain range of 63 dB, each stage of the two-stage feedback amplifier 200 covers about 31.5 dB. That is, the two-stage feedback amplifier 200 provides a gain of about 20*log(r2/r1)+20*log(r4/r3) which is equal to 31.5 dB+31.5 dB. Since the two-stage feedback amplifier 200 uses two operational amplifiers, more chip areas are used and more currents are consumed during the operation. In addition, two operational amplifiers can contribute more noise sources and degrade the noise performance and signal-to-noise ration (SNR).
From the foregoing, improvements to the conventional feedback amplifiers are desired.